1. Field of the Invention
The present invention relates generally to a metallization process for manufacturing semiconductor devices. More particularly, the present invention relates to the use of barrier layers having enhanced adhesion to overlying conductive films of copper and other conductive materials.
2. Background
Multilevel metal interconnects having a dimension smaller than 0.20 microns are expected to play a key part in achieving ultra large scale integration (ULSI), which is the next generation of very large scale integration (VLSI). It is also expected that the Damascene process, which involves the deposition of metal into patterned dielectric openings, followed by subsequent chemical-mechanical polishing (CMP) to provide planarization, will also play a key part in achieving such multilevel metal interconnects. As a result, there is a need for a method to reliably deposit metal into patterned dielectric trenches, and to do so in a way that leads to interconnects having desirable properties. The Damascene process is described in Ryu, C., xe2x80x9cMicrostructure and Reliability of Copper Interconnects,xe2x80x9d doctoral thesis, Stanford University (June 1998), which is hereby incorporated by reference.
Aluminum (Al) has been widely used as an interconnect metal because of its good electrical properties. Preferred, known procedures for depositing Al interconnects include chemical vapor deposition (CVD) and physical vapor deposition (PVD). CVD is a preferred procedure for depositing Al into high aspect ratio features of the kind found in Damascene processes, because it leads to good conformal layers of Al, i.e., layers that have a uniform thickness over the substrate surface even when the topography of the surface includes a base and sidewalls requiring step coverage, such as in a trench or contact via. It is known to fabricate Al interconnects by depositing Al by CVD at relatively low temperatures into apertures smaller than 0.5 microns.
However, as device sizes continue to shrink while device densities, chip sizes, and maximum interconnect length increase, the limitations of Al become increasingly apparent. In particular, interconnects having a width smaller than about 0.18 microns are desirable for the next generation of integrated circuits. However, at this dimension, the electromigration of aluminum can cause failures in the interconnect. The resistivity of Al also leads to unacceptably high resistances for long interconnects, which can lead to RC delay, i.e., a delay due to the time required for the energy stored in an interconnect to dissipate. Accordingly, new metals are needed to satisfy the requirements of the next generation of integrated circuits.
Copper (Cu) is currently being investigated as a replacement for aluminum in interconnects. Ryu, which was previously incorporated by reference, provides a review of the current state of the art with respect to copper interconnects. Cu has a bulk resistivity of 1.67 xcexcxcexa9-cm, which is approximately 40% less than that of Al (2.66 xcexcxcexa9-cm). Also, Cu exhibits resistance to electromigration superior to that of Al under similar circumstances, and lower RC delay. Thus, the lower resistivity of Cu accommodates a higher line density, i.e., a smaller width, while allowing for increased device speed.
Copper interconnects may be deposited by a variety of conventional procedures, such as physical vapor deposition (PVD), electroplating, and electroless plating. Chemical vapor deposition (CVD) is a viable method due to its superior step coverage and selective deposition capability. CVD involves the formation of a reaction product, copper in this case, on a substrate by thermal reaction or decomposition of gaseous compounds, referred to as precursors. Metal-organic CVD (MOCVD), which uses one or more organo-metallic precursors, is preferred for the CVD of copper because they may be used at relatively low temperatures. Preferred organo-metallic precursors include Cu+2(hfac)2 and Cu+2(fod)2, where hfac is an abbreviation for the hexafluoroacetylacetonate anion, and fod is an abbreviation for heptafluoro dimethyl octanediene.
A preferred process uses the volatile liquid complex copper+1(hfac)(tmvs) an a precursor, where tmvs is an abbreviation for trimethylvinylsilane, with argon as a carrier gas. Because this precursor is a liquid under ambient conditions, it can be utilized in standard CVD bubbler precursor delivery systems currently used in semiconductor fabrication. The deposition reaction is believed to proceed on a heated substrate according to the following mechanism, in which (s) denotes interaction with a surface and (g) denotes the gas phase.
(1) 2Cu+1(hfac)(tmvs)(g)xe2x86x922Cu+1(hfac)(tmvs)(s)
(2) 2Cu+1(hfac)(tmvs)(s)xe2x86x922Cu+1(hfac)(s)+2(tmvs)(g)
(3) 2Cu+1hfac(s)xe2x86x92Cu(hfac)(s)+Cu+2(hfac)2(s)
(4) Cu(hfac)(s)+Cu+2(hfac)(s)xe2x86x92Cu(s)+Cu+2(hfac)2(s)
In step 1, the precursor is adsorbed from the gas phase onto a metallic surface. In step 2, the precursor is dissociated to 2Cu+1(hfac) and 2 (tmvs). (tmvs) leaves the surface by desorption. In step 3, Cu(hfac) and Cu+2(hfac)2 are generated by electron exchange between surface Cu+1(hfac) species. In step 4, copper metal and volatile Cu+2(hfac)2 are formed by the migration of (hfac) groups. Cu+2(hfac)2 leaves the surface by desorption, leaving copper metal. The overall disproportionation reaction is described by the following equation:
2Cu+1(hfac)(tmvs)(g)xe2x86x92Cu(s)+Cu+2(hfac)2(g)+2(tmvs)(g)
Both tmvs and Cu+2(hfac)2 are volatile byproducts of the deposition reaction that are exhausted from the chamber. Cu+2(hfac)2 does not contribute to further deposition because the temperature is much lower than that required for Cu+2(hfac)2 decomposition.
Cu+1(hfac)(tmvs) can be used as a precursor to deposit Cu through either a thermal process, or a plasma based process, referred to as plasma enhanced CVD (PECVD). The substrate is preferably held at a temperature between about 100 and 400xc2x0 C. for PECVD of Cu from Cu+1(hfac)(tmvs). The substrate is preferably held at a temperature between about 150 and 220xc2x0 C., and more preferably at about 170xc2x0 C., for CVD of Cu from Cu+1(hfac)(tmvs) that is not plasma enhanced. Lower temperatures result in a very slow deposition rate, and higher temperatures may adversely affect the resistivity of the resultant interconnect. Thermal CVD is typically preferred over PECVD due to the lower temperatures typically involved with thermal CVD.
However, copper may diffuse into surrounding dielectric or insulating layers, as well as the underlying silicon substrate, and interfere with the desirable properties of those layers. This problem also exists with aluminum, and it is known to use a barrier layer to separate such interconnects from other features. Barrier layers for aluminum interconnects are commonly made from materials that include tantalum (Ta), tantalum nitride (TaN), titanium (Ti), and titanium nitride (TiN). It is also known to use a barrier layer to separate copper interconnects from other features. Barrier layers used to separate copper interconnects from other features include those listed above for use with aluminum interconnects. However, while the interaction between these barrier layers and aluminum has been intensively studied, the interaction with Cu may be different. In particular, there is often poor adhesion between barrier layers and the copper interconnects deposited on the barrier layers, which may lead to dewetting and device failures due to high via resistance and poor electromigration reaistance. This problem is particularly pronounced with Cu interconnects deposited by CVD, but may also exist to a lesser extent with Cu deposited by other methods, such as PVD, electroplating, and electroless plating. In addition, an improper selection of a barrier layer may lead to problems with the growth of the copper interconnect, interfacial contamination, and/or an undesirable microstructure in the copper. With respect to CVD, efforts at solving these problems have largely been directed to attempts to prevent chlorine and fluorine present in the precursors from incorporating into the copper films.
Layers of Cu deposited by PVD have typically demonstrated better adhesion to conventional barrier layers than layers of Cu deposited by CVD. However, CVD is preferred over PVD for other reasons, such as superior trench and via fill. To take advantage of the favorable properties of both CVD and PVD, it is known to deposit a seed layer of Cu by PVD for good adhesion to the underlying barrier layer, followed by the deposition of Cu by CVD to achieve superior trench and via fill. However, using both CVD and PVD requires extra process steps which increases manufacturing time and cost. It is also known to anneal CVD deposited Cu after deposition to enhance adhesion. See id.
The present invention provides a method for improving the adhesion of copper and other conductive metals to a substrate, such as a barrier layer. A barrier layer is provided that has a first surface that is substantially unoxidized. A copper layer is then deposited onto the first surface of the barrier layer. The substantially unoxidized state of the first surface enhances the adhesion of the copper layer to the barrier layer. The substantially unoxidized first surface of the barrier layer may be provided by preventing oxidation of the barrier layer subsequent to its deposition, or by removing or displacing oxidation from at least a portion of the barrier layer surface prior to deposition of the conductive metal. Further, an adhesion promoting material may be added to the barrier layer which ensures that at least a portion of the barrier surface remains free from oxidation. In the case of copper, the copper may be deposited by a variety of processes, including chemical vapor deposition (CVD), physical vapor deposition (PVD), electroless plating, and electroplating, for example.
The substantially unoxidized first surface of a barrier layer may be provided by including a noble metal in the barrier layer. This noble metal may be selected from the group consisting of gold (Au), silver (Ag), platinum (Pt), chromium (Cr), nickel (Ni), and palladium (Pd), for example. The barrier layer may consist essentially of the noble metal, or may be doped with the noble metal, so that at least a portion of the surface of the barrier layer will not be oxidized. The barrier layer may include an adhesion promotion layer of the noble metal. The noble metal may be added to the barrier layer by ion implantation and other techniques known in the art.
The substantially unoxidized first surface may also be provided by including a refractory metal that forms a volatile oxide at the barrier layer surface, using the techniques described above with reference to noble metals. This refractory metal may be selected from the group consisting of tungsten (W) and molybdenum (Mo), for example, but not by way of limitation.
The environment to which the barrier layer is exposed may also be controlled to minimize oxidation prior to application of the metal-comprising interconnect material. For example, the deposition of copper may be started while the deposition of the barrier layer is still proceeding. When the copper is deposited by CVD, the material of the barrier layer may be incorporated into the precursor during at least the first portion of the CVD deposition. Oxidation of the barrier layer during the deposition of copper by chemical vapor deposition (CVD) may be avoided by using a precursor that is substantially free of water.
The substantially unoxidized first surface may be provided by removing oxide from the surface of the barrier layer using techniques such as ion bombardment, chemical reaction to produce a volatile species, and contact with a displacing material, for example, and not by way of limitation.